Ebb and Flow were the twin space craft that mapped in the Moon's gravitational field by flying near each other, and then as the gravity of the Moon tugged on them they could suss out how much gravity that was, exactly. The gravity map of the moon, actually two of them, at two different scales, is done, so the space craft were "de-orbited."
To me, the first thing that is really interesting about this is the fact that they kept the space craft in very very low orbit for a long time. We earthlings tend to think of orbiting as something you have to do at high altitude, because we always send our satellites up high. But it is the atmosphere that requires that. With no atmosphere, an object can orbit a planet at very low altitude. Imagine that for a moment. Imagine that we form colonies on the moon and live there; there might well be regularly orbiting objects that would require that moon trucks stop for a moment at an intersection to allow them to pass. I'm not sure why such things would exist, but they could.
The mission scientists and engineers apparently had a great time flying ebb and flow down into low areas, pulling the two washing machine size science robots up just in time to miss hitting a ridge or crter wall or whatever, over the last several weeks. Getting in close to map gravity, I assume.
In the end, though, NASA was faced with needing to meet two objectives: 1) They can't leave stuff in orbit because that is messy. So they needed to "deorbit" the space robots; and 2) They did not want the craft to have any fuel on board at the moment of ... deorbiting. This is complicated because even though they have a very good idea of how much fuel is on board at any moment, there is some error in that measurement, and burning off the fuel for, say, 123 seconds vs. 126 seconds could make a huge difference in the final outcome.
GRAIL principal investigator Maria Zuber (left) and Bear Ride, the sister of late astronaut Sally Ride, appear together after the GRAIL twin spacecraft successfully complete their mission and impact the moon. Image credit: NASA/JPL-Caltech
If the space craft were far from the moon, you could just do that... the final speed of the space craft would not be too much of an issue. But since Ebb and Flow were at cropduster altitude (in space ship terms) a burn that would "point" the craft to the moon's flat surface would be very inaccurate. So, what they did instead was to drive Ebb and Flow into the side of a mountain. This way, the burn could be more or less time and that would cause a change in speed, but the craft would still hit the mountain. And, this is what NASA did this afternoon.
Right after the event, NASA announced that the people who are in charge of naming things approved the mission staff's request that the location of the crash-down be named after Sally Ride.
(And thus, the title of this post.)
The full NASA press release is here.
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Why are they concerned about onboard fuel when crashing into the moon? It's not like there's danger of a wildfire, or poisoning a watershed. Are they using hydrazine, or H2/O2?
There is at least one current mission, and presumably will be future missions, with the goal of mapping out surface volatiles on the Moon. Fuel, whether hydrazine or H2/O2, would contaminate those measurements.
From the press release: "Fifty minutes prior to impact, the spacecraft fired their engines until the propellant was depleted. The maneuver was designed to determine precisely the amount of fuel remaining in the tanks. This will help NASA engineers validate computer models to improve predictions of fuel needs for future missions. "
NASA also monitored the impact plumes with the LRO spacecraft's UV detector (LAMP) and wanted the GRAIL tanks depleted of volatiles to see the plume